In power plants, it is necessary to monitor the concentration of soluble silica compounds in the water fed to and in the boilers. Excessive amounts of such compounds (referred to hereinafter as "silica") can cause coating of the turbine blades which requires costly maintenance on a periodic basis. Typically, silica concentrations in excess of 20 parts per billion (ppb) are considered excessive.
Analyzers have been sold for continuously monitoring the concentration of silica and other chemical compositions in water and other fluids. One such device was manufactured and sold by Orion Scientific Instruments of Hawthorne, N.Y. as the Orion Model 1830 Silica Analyzer (hereinafter referred to as the Orion analyzer).
The Orion analyzer coupled a sample of water to be analyzed to a chemical cartridge where known reagents were added and mixed in proper sequence to yield a heteropoly blue complex, the intensity of which was proportional to the silica concentration. The heteropoly blue complex was then pumped to a flow cell positioned in a colorimeter of the type shown in U.S. Pat. No. 4,273,449 of Schmid entitled "Radiation Measuring Apparatus."
This colorimeter directed light through the flow cell and through a reference path so that adjustments for changes in the optical signals could be accommodated. In accordance with known procedures, a baseline value (corresponding to a zero silica content) and a full scale value could be entered into storage in a microprocessor which would then calculate a calibration curve.
The present invention is directed to an improvement over the automatic chemical analyzer disclosed in U.S. Pat. No. 5,230,863 which is incorporated by reference herein. As disclosed in the '863 patent, a computer controlled valving arrangement couples either the solution to be tested, a baseline solution, or a standard (full scale) solution to a chemical cartridge through a pumping mechanism which also feeds the desired reagents to the cartridge. In the preferred embodiment of the invention, the chemical under investigation is silica. Therefore, the desired reagents are: 1) sulfuric acid to acidify the solution; 2) ammonium molybdate to react with the acidified solution to yield a silicomolybdate complex; 3) oxalic acid to inhibit any further reaction from taking place and to prevent phosphate interference; and 4) ascorbic acid to reduce the silicomolybdate complex to a heteropoly blue complex. When the reaction is complete, the solution is directed to a flow cell which includes a chamber having walls through which the desired light source is passed to a detector such as a photodiode. The transmittance of the heteropoly blue complex is measured by the detector to determine the silica content of the solution. The baseline solution and standard (full scale) solutions are first passed through the flow cell automatically and the values of the light intensities for each is stored in memory in the computer. The computer calculates the calibration curve from these values so that thereafter, as the sample is tested, the voltages detected can be correlated within the computer to specific chemical concentrations.
The '863 patent establishes the baseline value by measuring the silica content of the baseline plus reagents solution, with and without the color reagent, the assumption being that the color reagent introduces silica contamination into the measured baseline solution. However, by diverting the color reagent away from the chemical module, and thus, away from the measuring chamber of the flow cell, the composition of the solution under investigation is concentrated resulting in a "dilution error." This "dilution error" may result in inaccurate baseline determinations. Further, the automatic chemical analyzer disclosed in the '863 patent did not fully account for silica contamination as a result of the reagents themselves.